Building better M2M devices through antenna optimization

The process of building a better M2M (machine-to-machine) device - one that not only fits the application scenario but also performs well in the field - starts with choosing the right antenna.

Selecting the right antennas for a device, particularly cellular antennas, and testing and integrating them properly, can prove the difference between a device that passes network certification and testing like PCS-1900 Type Certification Review Board (PTCRB) testing, and one that does not. Good radio frequency (RF) optimization and OTA (over the air) performance in wireless devices is critical for these certifications, and incorrect antenna selection and integration can often be blamed when M2M devices fail certification testing. Often, it is a system issue, such as the overall design of the system or how that device is interacting. As with many things, system optimization is the key to success.

This article discusses the important process of antenna selection, how to incorporate antennas in M2M devices and the recipe for connectivity success. First, let's take a look at selecting the proper antenna.

Antenna SelectionBetter antennas mean better devices. It’s as simple as that. Understanding the requirements for achieving specific network certification processes are the most important factors when first selecting a cellular antenna. Once the module provider and the carrier have been selected, the next part of the process should be selecting the right antenna for your application.

The first step is for the device maker and antenna company to partner from the start to incorporate their ideas for compliance and RF performance.

The second phase involves the device maker and antenna company cooperating on product design files to work out the advantages and disadvantages of each antenna configuration.

Once the antenna is selected, the customer can start to make prototypes of the device incorporating the required configuration for the recommended antenna. Depending on whether it is a custom design or not, the antenna partner either provides the device maker with a current production sample (off the shelf) or a handmade antenna for the prototype device that enables the device maker to finish their product design. When the product is finished and can connect to a network, the device maker sends the final device to the antenna partner.

Figure 1: The author in the Tagolas RF chamber

Next is the stage where active device optimization happens – this means the ability to increase the device send and receive sensitivity to improve transmissions. When devices are optimized it results in superior M2M devices. The process is what cell phone companies do in their own laboratories before sending devices to final certification. M2M companies in most cases do not have the resources to do that on their own, and they are required to choose antenna partners who can do it for them.

Selecting the right antenna can streamline the whole product design process and result in M2M devices passing certification testing the first time. In general, M2M systems are getting smaller to satisfy scenarios such as body-worn medical devices. Yet, the same device has to get excellent performance and will be compared to a cellphone. When devices are smaller, performance can be compromised, but using an antenna optimized for greater send and receive sensitivity can help compensate for size.

Antenna selection and integration will affect over-the-air (OTA) measurements and can affect radiated spurious emission (RSE) figures. Without high antenna efficiency, certain network OTA requirements, particularly total radiated power (TRP) will not be met. RSE is a common point of failure for M2M devices seeking PTCRB certification. This can be misinterpreted as an antenna issue. Here’s what can happen:

RSE failure can be caused by an antenna impedance mismatch with the module when the device is on and transmitting. The solution is to design the antenna for an active device, not just a passive device. There must be a good impedance match when the device is on and transmitting and when connected to the network / base station simulator.

When the antenna selection is good and efficiency is high, the TRP will be high. This is exactly what you want to achieve for optimal send and receive sensitivity. However, this can also result in the system re-radiating emissions - and with the increased power, RSE failures can result. It is not good practice to detune the antenna or bring down antenna efficiency to resolve this issue. The emission needs to be identified (the source) and eliminated, or at least prevented from getting to the antenna and being received into the system